1. Field of the Invention
The present invention relates to a light emitting device using an organic compound material as a luminescent material, and in particular, to the configuration of a light emitting device in which a plastic material is used as a substrate.
2. Description of the Related Art
Heretofore, a light emitting device utilizing a luminescence phenomenon with electroluminescence (hereinafter, referred to as EL) has been investigated so as to be applied as a means for forming image that represents information such as characters and images. Among inorganic compound materials, zinc sulfide has been known as a typical luminescent material (EL material) that generates EL. On the other hand, among organic compound materials, an aluminum tris(8-hydroxyquinoline) (Alq3) complex has been known as a typical EL material.
The light emitting element using such an EL material has a comparatively simple structure and contains a luminescent material included in a coating film of several hundreds nanometers between a pair of electrodes. From the view point of diversity of the material, it is considered that the luminescent material containing the organic compound material is superior to one containing the inorganic compound material. Initially, it was concerned that the life of light emission was short. In recent years, however, an organic compound material having a brightness half life of over 10,000 hours has been developed.
The light emitting device adapted to various kinds of applications such as a lighting means or a displaying means by the use of a light emitting element obtains additional advantages of weight saving, low-profiling, resistance to breakage, flexibility, and so on. Therefore, the values as a charming product can be expected.
However, there is a problem in that the plastic material has low heat resistance properties compared a glass material. Even though such a problem can be overcame by decreasing the process temperature, it is difficult to implement a light emitting device having such a plastic material as a substrate because the plastic material has the property of allowing the transmission of oxygen and moisture content, and the property of occluding oxygen and moisture content and releasing them again depending on the temperature. In other words, there is a problem that the organic compound material or which is one of the structural components of the light emitting device or an anode material using the alkali metal can be influenced and the light-emitting function thereof can be decreased.
For solving such a problem, for example, the following methods are disclosed. That is, a method of using a material having high moisture barrier properties such as a fluorine film as a plastic material, a method of forming a thin film layer containing one or two or more of materials selected from metal fluorides and magnesium oxides essentially containing silicon oxide as disclosed in JP8-167475, a method of forming a gas-barrier layer by laminating inorganic nitride and inorganic oxide as disclosed in JP8-68990. In the U.S. Pat. No. 6,268,695, there is disclosed a laminate structure as a barrier layer in which polymer layers and ceramic layers are laminated one after another.
However, the fluorine film is expensive and a sufficient thickness of the film is required for attaining sufficient gas-barrier properties. As a result, there is a problem of lowering transmissivity. In addition, the coating film made of the inorganic compound such as nitride or oxide has a high internal stress, so that the plastic substrate can be deformed when the coating film is thickened. On the other hand, if the coating film is thinned, a sufficient gas barrier property cannot be obtained and a pin hole or the like tends to be caused. Therefore, there is a problem in that a sufficient effect as a sealing material cannot be obtained. Furthermore a fine film made of silicon oxide or silicon cannot be obtained if the material is not heated to the heat-resistance temperature or more of the plastic material. On the other hand, when the temperature decreases, the coating film becomes coarse and the gas barrier property decreases. These features are mutually contradictory, so that the plastic substrate cannot be coated with a fine film.
Furthermore, silica, alumina, titanium oxide, indium oxide, tin oxide, indium tine oxide (ITO, indium oxide mixed with tin oxide), aluminum nitride, silicon nitride, or the like selected as a ceramic layer has brittleness, so that it may be not always preferable to use a flexible plastic material as a substrate. In addition, it becomes possible to endure some degree of bending if the ceramic layer is formed as a thin film. In this case, however, the probability of generating a pin hole increases and so on, resulting in a decrease in gas barrier property.